Various processes for making reconstituted tobacco are known in the art. Many of these processes include an aqueous extraction of the tobacco plant parts followed by treatment of the extract and subsequent recombination of the thus treated extract with tobacco pulp. A particularly preferred treatment of the tobacco extract involves removal of some of the inorganic constituents from the extract prior to its recombination with the fibrous tobacco pulp. Potassium nitrate removal is particularly desirable for several reasons. First, the burn rate of the tobacco products will be diminished; and secondly, some of the products of combustion, such as oxides of nitrogen, are reduced. Furthermore, the reconstituted tobacco will have a lowered bulk density per unit weight and an improved filling capacity.
Removal of constituents present in aqueous tobacco extracts has been dealt with for many years, and various methods have been proposed. For example, U.S. Pat. No. 720,830 to Marsden describes a method for treating an aqueous tobacco extract by subjecting the extract to heating under pressure so as to flash off the liquid constituents such as "fusel oil." The solid residue is dissolved in water, and the mixture is boiled until the mineral matter consisting of sodium nitrate, some of the potassium nitrate, and other mineral matter crystallizes out and is separated from the liquor. The Marsden patent fails to describe or suggest an important aspect of the present invention which is to recover potassium nitrate in a relatively pure and useful form. In addition, the use of heat in excess of 250.degree. F. in the Marsden process results in the loss of many desirable volatile tobacco flavorants in the flash distillation step and the subsequent boiling of the extract.
U.S. Pat. No. 3,428,053 describes a centrifugation step which removes a significant amount of the solid insoluble constituents from the aqueous extract prior to concentration and reapplication to the tobacco sheet. The identity of the thus separated solids was not elucidated; however, it is believed that very little, if any, of the water-soluble potassium nitrate could be removed or recovered by using this method.
U.S. Pat. Nos. 3,616,801 and 3,847,164 describe methods wherein ion exchange and ion retardation resins are utilized to selectively remove inorganic constituents and are specifically directed to the removal of potassium nitrate from aqueous extracts of tobacco. However, no attempt was made to recover the potassium nitrate in a useful form. These particular methods may be feasible on a small scale but are apt to be both costly and cumbersome on a practical commercial scale. In addition, regeneration of the ion exchange resin or disposal of the resin containing the crude potassium nitrate and other undesirable elements adds to the cost and also presents a problem from an ecological and environmental viewpoint.